1. Field of the Invention
The present invention is directed to a fluid pump control device for wheel loaders and, more specifically, to a fluid pump control device for wheel loaders capable of distributing and controlling the output power of an engine in an optimized manner depending on the magnitude of a hydraulic working load and a travel load.
2. Description of the Related Art
Wheel loaders, one kind of heavy construction equipments, includes a single engine to which are operatively connected an axle and a fluid pump in such a manner that the engine provides both a travel power for rotating the axle and a hydraulic working power for driving a fluid pressure generating apparatus, e.g., fluid pump, that actuates front working implements such as a boom, an arm, a bucket and the like.
More specifically, as shown in FIG. 1, an engine 8 of wheel loaders is connected to an axle through a transmission 9 to drive a vehicle forwards or rearwards and also associated with a main pump P1, a steering pump P2 and an auxiliary pump P3. Accordingly, the output power of the engine 8 is split into an axle driving power and a hydraulic working power.
The main pump P1 is designed to discharge and supply a hydraulic flow to actuators for operating front working implements such as a boom, an arm, a bucket and the like. The steering pump P2 generates a hydraulic flow which is either to be supplied to a steering device 7 or merged with the hydraulic flow from the main pump P1 and then supplied to the front working implements.
The auxiliary pump P3 feeds a hydraulic flow to a brake device 12 and a joystick valve 15. Responsive to fore-and-aft or side-by-side motion of a manipulation lever 15a, the joystick valve 15 is adapted to reduce the pressure of the hydraulic flow supplied from the auxiliary pump P3 and then feed pilot signal pressures to a control valve 1 that serves to control movement of the front working implements.
As briefly noted above, the prior art wheel loaders are designed to divide the output power of the engine into a power required for travel and a power for driving fluid pressure generating devices, including the main pump P1, the steering pump P2 and the auxiliary pump P3.
Hydraulic control devices of the conventional wheel loaders are classified into two types depending on how to control the hydraulic flows discharged by pumps in case that a hydraulic working load and a travel load take place simultaneously as in the process of performing the tasks of hydraulic working and traveling at the same time. One is a combination type hydraulic control device that merges a hydraulic flow generated by a steering pump with a hydraulic flow discharged by a main pump to enable them to cooperatively undertake the hydraulic working load. The other is a self-reliance type hydraulic control device that allows the main pump hydraulic flow to undertake the hydraulic working load, while permitting the steering pump hydraulic flow to undertake the travel load.
In the combination type hydraulic control device as shown in FIG. 1, which makes use of the hydraulic flows discharged by the main pump and the steering pump, the output power of an engine is distributed to the side of the hydraulic working load in the first place. This helps to improve the performance of dealing with the hydraulic working load but may result in a reduced travel torque.
On the other hand, the self-reliance type hydraulic control device provides an advantage in that a sufficiently great travel torque can be acquired even when the tasks of traveling and working are performed simultaneously. However, the self-reliance type hydraulic control device is inferior in terms of the hydraulic working performance because the hydraulic flow of the main pump alone is used to deal with the hydraulic working load.
In a nutshell, the two types of hydraulic control devices employed in the prior art wheel loaders are focused on one of the tasks of hydraulic working load and traveling and therefore fail to exhibit satisfactory performance with respect to the other task.